Spectral analyzers have long gained favor because they provide a fast and non-destructive means of analyzing different types of samples. Based on this technology, it is possible not only to determine the characteristics of a sample surface, but sometimes the constituent components beneath a sample surface. Typically, an optimal range of wavelengths is selected to irradiate a sample, where reflected or transmitted light is measured to determine the characteristics of the sample. Some samples, for example, are best analyzed using a near infrared spectrum of light while others are optimally analyzed using a range such as visible or mid infrared spectrum.
Analyzers of the prior art typically use a filter wheel or scanning diffraction grating to serially generate the specific wavelengths that are of interest in analyzing a sample. Based on moving parts, filter wheels and scanning diffraction gratings are sensitive to vibration and are not reliable in analyzing a sample generally. They therefore are not suitable for withstanding the mechanical vibrations generated by machinery, and therefore have not found use in real-time measurements of samples other than in controlled laboratories.
Optical systems typically include fiber optic cables to conveniently transmit light from a source to a destination located at a distance. Unfortunately, fiber optic cables can not be used in certain applications, such as those that include machinery generating severe mechanical vibrations, without further conditioning of the optical signal because such mechanical vibrations can cause undesirable modal disturbances within the optical fiber. These modal disturbances create light intensity disturbances that are not related to the surface or internal properties of a sample. Therefore, without incorporating costly conditioning mechanisms, the quality of an optical signal can be degraded. This detracts from the accuracy of the spectral measuring device.
Most spectral analyzers utilize a narrow spot size to intensely irradiate a sample to be analyzed. This is largely due to the fact that most wavelength detectors for analyzing a sample depend on reflected light that is transmitted through a fiber optic cable. Illuminating a sample with a highly intense incident light typically results in a greater amount of reflected light that is more easily measured by a detection device, which is often limited in sensitivity. Unfortunately, a narrow spot size can sometimes provide inaccurate measurements because a small spot may not be representative of the whole sample.
Some spectral analyzers further include an illuminating source disposed in the same cavity as a detector that receives the reflected light from an irradiated sample. In such a case, stray light reflecting from within the chamber, rather than off the sample, is sometimes erroneously included in the measurement. This often has a devastating impact on measurement accuracy.
The overall design of a spectral analyzer, therefore, including its individual components is critical to provide the most accurate method of detecting subtle differences in an analyzed sample. The balance of this specification discusses the features of the inventive spectral analyzer and associated methods in detail.
This invention is a spectral analysis system and method for determining percentage concentration of constituents and color characteristics of a sample. It has a wide array of applications in areas that require spectral measurements of larger sample areas. Such applications include but are not limited to non-invasive blood analysis, surface moisture measurements, and calorimeter analysis of a samples such as wallpaper.
The invention uses the diffuse reflectance properties of light to obtain percentage concentrations of constituents in samples such as agricultural products or blood. Additionally, the invention uses the diffuse reflectance properties of light to determine color components of a sample area such as a section of wallpaper or paint on an automobile.
In the preferred embodiment, techniques of the present invention involve measuring a spectral response to various wavelengths from visible to infrared. Typically, a preferred range of wavelengths is determined for a particular application and a corresponding detection device and illuminating lamp are matched accordingly for the application. Visible wavelength light is ordinarily used in calorimeter applications, while infrared is preferably used in grain monitoring applications.
The analyzer of the present invention includes a light source having a suitably broad bandwidth for simultaneously irradiating a sample to be analyzed with multiple wavelengths of light. A detector receives the radiation diffusely reflected from the sample where the received optical signal is analyzed by a real-time computation subsystem to determine constituents or color components of the sample.
A light source is angularly positioned in a first chamber to irradiate the sample through a window formed of a suitable protective material such as sapphire or glass. Optionally, the light source is focused using a lens or parabolic mirror to intensify the light irradiating the sample. This enhances reception of reflected light off the sample into the detector, which is positioned in a second chamber. The design of each chamber ensures that stray light from the lamp is not received by the detector from within the detection apparatus itself during a sample measurement. Rather, light received by the detector positioned in second chamber, adjacent to the first chamber, is essentially only light reflected off the sample.
In the preferred embodiment, the windows are separated from each other on a common plane while associated chambers are adjacent to each other. However, the windows are optionally angular with respect to a common plane. Further, the light source and detector in the first and second chamber respectively are optionally positioned across from each other such that light transmits through the sample into the detector. In this embodiment, a spectral analysis is then performed on the transmissive properties of the sample instead of reflective properties.
The second chamber includes a diffuser in the path of the light received from the irradiated sample to ensure that only spectral information is measured without imaging of the sample. The diffused optical signal emanating from the diffuser is then fed into a wavelength separator, such as a linear variable filter (LVF), within the second chamber to spatially separate the wavelengths of interest.
The wavelength separator in turn feeds the optical signal into a suitable detection device, such as a multiplexed detection array, which is capable of simultaneously detecting the spatially separated wavelengths reflected from the irradiated sample. Electrical signals from the detection device corresponding to individual wavelengths of light from the irradiated sample are converted into digital data where they are spectrally analyzed by a computation device to calculate color components or the percentage concentration of various constituents of the sample.
The present invention also includes a reflective device in the first chamber to redirect a portion of the optical lamp light, which serves as an optical reference, into the detector located in the second chamber. A controllable shutter mechanism is used to block this reference light when a sample is spectrally analyzed. Conversely, another shutter mechanism blocks light reflected from the sample when the reference light is spectrally analyzed. Based on a combination of reference and sample measurements, a precise wavelength analysis is used to determine, for example, constituents in a sample such as blood or the average color of a section of wall paper for paint color matching.
The components comprising the present invention are preferably integrated into a single unit to create a portable handheld spectral analyzer capable of illuminating a sample with a large spot size, where reflected light is further detected using a wide angle viewing aperture. Such a device is beneficial in applications where a sample cannot be easily moved to the analyzer. For example, a piece of wallpaper adhered to the surface of a wall in a home can be analyzed by focusing the handheld portable analyzer on the desired area and measuring reflected wavelength properties.
The analyzer of the present invention advantageously monitors a sample without requiring an expensive and restrictive fiber optic cable. Modal disturbances caused by mechanical vibrations in the optical fibers are therefore avoided. Furthermore, the aperture of monitored light from the irradiated sample can be much larger because there is no need to incorporate an optical pickup to guide the sample light into a narrow fiber optic cable. The wide aperture optical return signal results in a larger analyzed sample area supporting more accurate sample measurements.